I. Field of the Invention
This invention relates to procedures for sampling molten materials, e.g. molten metals and salts, for purposes such as analysis or testing. More particularly, the invention relates to such procedures intended for obtaining small samples that are representative in terms of the composition of the bulk of the molten material.
II. Description of the Prior Art
It is often important in many industries to test the compositions of pools of molten materials during manufacturing processes, e.g. for process control or for certification of materials sold to customers. For this purpose, small samples of the materials are usually extracted and allowed to solidify and then the solids are analyzed directly, or after being dissolved in suitable solvents or liquid reactants, by techniques such as spectroscopy. Many such techniques test only very small volumes of the sampled material, so if the sample actually tested is not fully representative of the composition of the bulk material, the test results may be inaccurate. For example, optical emission spectroscopy uses a spark source to volatilize a small surface portion of a solid sample and the spectrum of the volatilized material is then determined. Similarly, inductively coupled emission spectroscopy uses a plasma to vaporize and excite a solution containing a metal sample dissolved acidic solution at a concentration of about 1 to 2%.
Unfortunately, it is extremely difficult in practice to obtain a solid sample from an homogenous pool of molten material that is itself fully homogenous and accurately representative of the bulk material. This is because there is a significant segregation of the constituent elements when large quantities of many molten mixtures are allowed to solidify relatively slowly. This is particularly true when the molten material is a mixture of two or more components, e.g. alloys or salt mixtures, however, inhomogeneities can also materialize when materials that are considered to be single components are cooled. This is because commercial industrially-produced metals and salts are rarely pure and are thus rarely single-components systems. Moreover, even relatively pure single component systems may cool to form inhomogeneous crystallographic regions, e.g. regions having crystals of different sizes or crystal structure due to differential rates of cooling. Such metals may therefore not be homogenous to the metallographer or crystallographer, even though samples may be homogeneous from the bulk chemical perspective.
In conventional sampling techniques involving the pouring of a relatively large volume of the molten material into a cool mould to form a disc-like or cylindrical sample typically weighing 25 to 160 g or more (when the material is a metal), each alloying or constituent element present in the molten material tends to migrate at a different distinctive rate from the colder outside part of the sample to the warmer inner part. This causes quite significant concentration contours or profiles in the sample, when it is fully solidified. The differential distribution of elements along the lengths and widths of these samples makes it an extreme challenge to select an area of a sample that should be analyzed to ensure representative results. To ensure the highest accuracy, it would be necessary to analyze the entire sample, but this would involve vaporizing or dissolving the total sample weight. Clearly, vaporization of a large sample cannot be achieved using rapid or economic means. Dissolving an entire sample, in the case of a metal, involves machining the sample to reduce it to small chips or turnings, and then dissolving the entire mass in acid or the like. This is an extremely slow and expensive process.
This problem could be reduced if samples of the molten material could be extracted and cooled fast enough to avoid significant partitioning of the melt, but molten materials, and particularly molten metals and salts, are difficult to handle quickly and accurately because of their problematic fluid properties, high temperatures and high latent heats of solidification that tend to slow cooling times.
Various sampling devices are known for removing test samples of molten materials, but they either produce samples that are too large for modern testing methods, and are hence slow to cool, or they are cumbersome and inefficient. Some of these known test devices are discussed below as typical examples of what is already known.
Edward J. Kelsey in U.S. Pat. 4,007,641 issued on Feb. 15, 1977 describes a molten metal sampler having a chamber that can be evacuated and that is adapted to hold the vacuum until the sampler is used. When the sampler is contacted by molten metal, a seal holding the vacuum disintegrates to permit passage of molten metal into the chamber where the metal cool and solidifies to form a large disc or pin shape. However, the large sample is allowed to cool naturally and so the homogeneity of the sample is unlikely to be acceptable. Moreover, the device is inconvenient to use because it has to be evacuated using some kind of pumping device and then sealed before it can be used.
Kazuo Moriya in U.S. Pat. 4,179,931 discloses a similar concept in which molten metal is drawn into an evacuated insulated paperboard cartridge through an elongated fill tube secured to the side of the cartridge. A separate cumbersome pumping device is required to evacuate the cartridge and natural cooling of the sample takes place.
In British patent application 2 086 040 A published on May 6, 1982, Cottam et al. disclose a molten metal sampler consisting of a uniform bore silica tube open at both ends and surrounded along its length by a sleeve of molten refractory. The sample is obtained by immersing one end of the tube into the melt for a period of time (five or six seconds) sufficient to allow the molten material to freeze in the bore. The tube is extracted and water-quenched and then the sample, in the form of relatively thin rod or pin, is retrieved by breaking the tube. Even though the samples are quite small, cooling and solidification may be slow enough to produce non-homogenous samples.
Finally, Vieth et al. in Analytical Chemistry, 1992, 64, 2958-2964 discloses a procedure to convert molten gallium into solidified pins for analysis. The procedure involves inserting a small TEFLON.RTM. tube of 2.5 mm outside diameter into the melt, drawing the melt up into the tube by undisclosed means, removing the filled tube, quenching the tube and solidifying the contents by plunging the tube into a refrigerant such as liquid nitrogen. This procedure is clearly inconvenient because of the need for specialized refrigerant and, moreover, since solidification may commence before the tube is plunged into the refrigerant, the homogeneity of the resulting sample may not be acceptable.
There is accordingly, a need for an improved method and apparatus for sampling molten material such as metal alloy or salt mixtures.